Due to its characteristics of being easily applicable to various products and electrical characteristics such as a high energy density, a secondary battery is not only commonly applied to a portable device, but universally applied to an electric vehicle (EV), a hybrid electric vehicle (HEV), or an energy storage system that is propelled by an electric motor. This secondary battery is gaining attention for its primary advantage of remarkably reducing the use of fossil fuels and not generating by-products from the use of energy, making it a new eco-friendly and energy efficient source of energy.
A secondary battery can be charged and discharged repeatedly by electrochemical reactions between elements including a cathode current collector, an anode current collector, a separator, an active material, an electrolyte solution, and the like. By way of example, a lithium polymer secondary battery being widely used has an operating voltage in a range of about 3.7V to about 4.2V. Accordingly, to obtain a high power battery pack for use in an electric vehicle, a plurality of unit secondary battery cells are connected in series to construct a battery pack.
In addition to this basic structure, the battery pack further includes a battery management system (BMS) to monitor and control a state of a secondary battery by applying an algorithm for control of power supply to a driving load such as a motor, measurement of electrical characteristic values such as current or voltage, charge/discharge control, voltage equalization control, state of charge (SOC) estimation, and the like.
Recently, with the increasing need for a high-capacity structure as well as utilization as an energy storage source, the demand for a battery pack of a multi-module structure in which a plurality of battery modules including a plurality of secondary battery cells are assembled, is also increasing.
Because the battery pack of the multi-module structure includes a plurality of secondary battery cells, there is a limitation in controlling the charge/discharge state of all the secondary battery cells or the plurality of battery modules using a single BMS. Accordingly, a recent technology has been introduced in which a battery management unit (BMU) is provided to each battery module included in the battery pack, the BMUs are designated as a slave unit, and a master BMS is additionally provided to control the slave units, such that the charge and discharge of each battery module is controlled in a master-slave mode.
FIG. 1 is a block diagram schematically illustrating a connected state of an external device 10 and a BMS 100 according to a related art.
Referring to FIG. 1, the BMS 100 including eight slave units 210 and one master unit 211 is provided. Also, the BMS 100 is connected to a battery pack 110 including a plurality of secondary battery cells 111 connected in series. The BMS 100 controls the charge and discharge of the battery pack 110, and further, measures the voltage of the secondary battery cells 111 included in the battery pack 110. The BMS 100 receives a control signal associated with charge and discharge from the external device 10, and transmits data associated with a state of the secondary battery cells 111. However, as described in the foregoing, because the battery pack 110 includes the plurality of unit secondary battery cells connected in series to obtain a high power battery pack for use in electric vehicles and the like, the external device 10 needs to be electrically separated from the BMS 100 to prevent the external device 10 from being damaged due to a high voltage.
To this end, a photo coupler capable of transmitting and receiving an electrical signal while electrically separating the external device 10 from the BMS 100 is connected and used.
The photo coupler is also called an opto-coupler. The photo coupler corresponds to a switching element including a light emitting source (input) and a light detector (output). Generally, an infrared light emitting diode (LED) is used as the light emitting source, and a photodiode or a phototransistor which turns on in response to light is used as the light detector. Thus, when an electric current flows to the input side, the light emitting source emits light, and then the output side element, i.e., the photodiode or phototransistor turns on. That is, the photo coupler is a switching element designed to turn on and off by light, not electrical coupling.
When the photo coupler is used in connecting the external device 10 to the BMS 100, an advantage is that the external device 10 may be electrically separated from the BMS 100. Also, a reverse current causing a high voltage current from the battery pack 110 to be inputted to the external device 10 side may be prevented while transmitting a data signal, and influence of electromagnetic waves generating during charge and discharge of the battery pack 110 may be lessened.
When the BMS operates in a master-slave mode, to provide a precaution against a failure that may occur in the master unit 211, the slave units 210 may be configured to directly deliver information associated with the secondary battery cells 111 to the external device 10. FIG. 1 illustrates three types of data each slave unit 210 transmits to the external device 10 to take precaution against a failure in the master unit 211.
A ‘2nd_PROT’ signal transmitted from the slave unit 210 refers to a signal representing that the secondary battery cells 111 are over-charged more than a preset voltage. A ‘Under V PROT’ signal transmitted from the slave unit 210 refers to a signal representing that the secondary battery cells 111 are over-discharged less than a preset voltage. A ‘Diag’ signal transmitted from the slave unit 210 refers to a signal representing whether an abnormality has occurred or not through self-diagnosis of the slave unit 210.
In this way, each slave unit 210 transmits three types of data to the external device 10, and when a number of such slave units 210 is eight, a total number of photo couplers needed to connect the external device 10 to the slave units 210 is twenty four. The photo coupler is a more costly element than other electric and electronic elements, which is a factor of increasing a manufacturing cost of the BMS.
Therefore, there is a need for studies on a BMS that may transmit a signal from the slave units 210 included in the BMS 10 to the external device 10 while maintaining insulation between the BMS 100 and the external device 10.